This invention relates to a composition and a method of improving the properties of power transmitting fluids, particularly to obtaining automatic transmission fluids of improved viscosity control and anti-shudder durability.
Automatic transmissions continue to become more sophisticated in design as vehicle technology advances. These design changes result from the need to improve vehicle operability, reliability, and fuel economy. Vehicle manufacturers worldwide are increasing vehicle warrantee periods and service intervals on their vehicles. This means that the automatic transmission and the automatic transmission fluid (ATF) must be designed to operate reliably without maintenance for longer periods of time. In the case of the fluid, this means longer drain intervals. To improve vehicle operability, especially at low temperature, manufacturers have imposed strict requirements for fluid viscosity at -40.degree. C. To cope with longer drain intervals and more severe operating conditions, manufacturers have increased the requirements for oxidation resistance of the ATF and increased the amount of wear protection that the fluid must provide for the transmission. To improve the fuel economy of the vehicle and reduce energy loss in the torque converter, manufacturers employ continuously slipping torque converter clutches which require very precise control of fluid frictional properties. A common element in the quest for better reliability, longer service life, and better transmission control is the viscometric properties of the fluid.
One method of improving overall vehicle fuel economy used by transmission designers is to build into the torque converter a clutch mechanism capable of "locking" the torque converter. "Locking" refers to eliminating relative motion between the driving and driven members of the torque converter so that no energy is lost in the fluid coupling. These "locking" or "lock-up" clutches are very effective at capturing lost energy at high road speeds; however, when they are used at low speeds vehicle operation is rough and engine vibration is transmitted through the drive train. Rough operation and engine vibration are not acceptable to drivers.
The higher the percentage of time that the vehicle can be operated with the torque converter clutch engaged, the more fuel efficient the vehicle becomes. A second generation of torque converter clutches have been developed which operate in a "slipping" or "continuously sliding mode". These devices have a number of names, but are commonly referred to as continuously slipping torque converter clutches. The difference between these devices and lock-up clutches is that they allow some relative motion between the driving and driven members of the torque converter, normally at relative speeds of 10 to 100 rpm. This slow rate of slipping allows for improved vehicle performance as the slipping clutch acts as a vibration damper. Whereas the "lock-up" type clutch could only be used at road speeds above approximately 50 mph, the "slipping" type clutches can be used at speeds as low as 25 mph, thereby capturing significantly more lost energy. It is this feature that makes these devices very attractive to vehicle manufacturers.
It is well known that lowering the viscosity of an ATF at low temperatures (e.g., -40.degree. C.) will result in improved operability of the transmission at low ambient temperatures, that increasing the amount of antiwear additives in the ATF will result in more wear protection, and that better friction control can be obtained by judicious choice of friction modifiers. However, we have now found that by proper selection of viscosity modifier molecular weight and the particular friction modifiers used, the low temperature operability, service life, and friction control of the ATF can be improved simultaneously.
Correct choice of the viscosity modifier molecular weight allows the fluid to meet the high temperature viscosity requirements imposed by the manufacturer while also allowing the fluid to meet rigorous low temperature viscosity limits. High temperature viscosity is also known to control wear in hydrodynamic and elastohydrodynamic wear regimes. High initial viscosity, at high temperatures (e.g., 100.degree. C. and 150.degree. C.), at both low shear (i.e., 1 to 200 sec..sup.-1) and high shear rates (e.g., 1.times.10.sup.6 sec..sup.-1) helps to control wear in hydrodynamic lubrication situations. Equally important is the fluid's ability to maintain this viscosity under both high and low shear rates even after use. High initial viscosity at high temperatures and low shear rates is important to transmission operability. High viscosity at high temperature and low shear rate controls fluid leakage at high pressures. This is not leakage from the transmission itself, but leakage at high pressures (e.g., 830 kPa (120 psi)) around seals and valves in the transmission control system. No matter how sophisticated the electronic control of the transmission, if the fluid is leaking under pressure in the valve body, the transmission will not function properly. This is particularly important in transmissions using sliding torque converter clutches since control of these devices is accomplished via minute fluctuations in clutch actuating pressure.
We have found that by careful selection of the molecular weight of the viscosity modifier in the presence of selected friction modifiers, the aforementioned properties of the ATF can be improved simultaneously. If the molecular weight of the viscosity modifier is too low, too much viscosity modifier will be needed to produce the required viscosity at high temperatures. This is not only uneconomical but will eventually cause elevation of the viscosity at low temperature making it difficult, if not impossible, to meet lower -40.degree. C. Brookfield viscosities. If the molecular weight of the viscosity modifier is too high, it will degrade by both mechanical shear and oxidation during service such that the high temperature viscosity contributed by the polymer will be lost, making the transmission vulnerable to wear and internal leakage. Adding sufficient high molecular weight polymer to give the required "used oil viscosity" causes elevation of the low temperature Brookfield viscosity of the fluid, possibly exceeding the specified maximum viscosity.
Since fluids exhibiting the characteristics of this invention must have exceedingly good low temperature fluidity (e.g., Brookfield viscosity .ltoreq.15,000 centipoise (cP) at -40.degree. C.), careful selection of the lubricant base oil is required. The use of certain highly refined mineral oils permits formulators to achieve the desired Brookfield viscosity without including synthetic materials. When using base oils with poorer low temperature characteristics, however, it may be necessary to use a lubricating oil that contains a synthetic base oil.
Continuously slipping torque converter clutches impose very exacting friction requirements on automatic transmission fluids used with them. The fluid must have a very good friction versus velocity relationship, i.e., friction must always increase with increasing speed. If friction decreases with increasing speed, a self-exciting vibrational state can be set up in the driveline. This phenomenon is commonly called "stick-slip" or "dynamic frictional vibration" and manifests itself as "shudder" or low speed vibration in the vehicle. Clutch shudder is very objectionable to the driver. A fluid which allows the vehicle to operate without vibration or shudder is said to have good "anti-shudder" characteristics. Not only must the fluid have an excellent friction versus velocity relationship when it is new, but the fluid must retain those frictional characteristics over the lifetime of the fluid, which can be the lifetime of the transmission. The longevity of the anti-shudder performance in the vehicle is commonly referred to as "anti-shudder durability". It is this aspect of fluid frictional performance that this invention addresses.
It has previously been found that certain compounds made by reacting isomerized alkenyl substituted succinic anhydrides (and their saturated alkyl analogs) with polyamines, when used with overbased metallic detergents, provide a unique solution to the problem of extending anti-shudder durability (see U.S. Ser. No. 837,639 filed Apr. 21, 1997). We have now found that when these friction modifiers are used in fluids of improved viscometric properties, automatic transmission fluids of significantly improved overall performance result.